patch-2_6_7-vs1_9_1_12

[linux-2.6.git] / drivers / scsi / scsi_lib.c

/*
 *  scsi_lib.c Copyright (C) 1999 Eric Youngdale
 *
 *  SCSI queueing library.
 *      Initial versions: Eric Youngdale (eric@andante.org).
 *                        Based upon conversations with large numbers
 *                        of people at Linux Expo.
 */

#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/completion.h>
#include <linux/kernel.h>
#include <linux/mempool.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/pci.h>

#include <scsi/scsi_driver.h>
#include <scsi/scsi_host.h>
#include "scsi.h"

#include "scsi_priv.h"
#include "scsi_logging.h"


#define SG_MEMPOOL_NR           (sizeof(scsi_sg_pools)/sizeof(struct scsi_host_sg_pool))
#define SG_MEMPOOL_SIZE         32

struct scsi_host_sg_pool {
        size_t          size;
        char            *name; 
        kmem_cache_t    *slab;
        mempool_t       *pool;
};

#if (SCSI_MAX_PHYS_SEGMENTS < 32)
#error SCSI_MAX_PHYS_SEGMENTS is too small
#endif

#define SP(x) { x, "sgpool-" #x } 
struct scsi_host_sg_pool scsi_sg_pools[] = { 
        SP(8),
        SP(16),
        SP(32),
#if (SCSI_MAX_PHYS_SEGMENTS > 32)
        SP(64),
#if (SCSI_MAX_PHYS_SEGMENTS > 64)
        SP(128),
#if (SCSI_MAX_PHYS_SEGMENTS > 128)
        SP(256),
#if (SCSI_MAX_PHYS_SEGMENTS > 256)
#error SCSI_MAX_PHYS_SEGMENTS is too large
#endif
#endif
#endif
#endif
};      
#undef SP


/*
 * Function:    scsi_insert_special_req()
 *
 * Purpose:     Insert pre-formed request into request queue.
 *
 * Arguments:   sreq    - request that is ready to be queued.
 *              at_head - boolean.  True if we should insert at head
 *                        of queue, false if we should insert at tail.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       This function is called from character device and from
 *              ioctl types of functions where the caller knows exactly
 *              what SCSI command needs to be issued.   The idea is that
 *              we merely inject the command into the queue (at the head
 *              for now), and then call the queue request function to actually
 *              process it.
 */
int scsi_insert_special_req(struct scsi_request *sreq, int at_head)
{
        /*
         * Because users of this function are apt to reuse requests with no
         * modification, we have to sanitise the request flags here
         */
        sreq->sr_request->flags &= ~REQ_DONTPREP;
        blk_insert_request(sreq->sr_device->request_queue, sreq->sr_request,
                           at_head, sreq, 0);
        return 0;
}

/*
 * Function:    scsi_queue_insert()
 *
 * Purpose:     Insert a command in the midlevel queue.
 *
 * Arguments:   cmd    - command that we are adding to queue.
 *              reason - why we are inserting command to queue.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
 * Returns:     Nothing.
 *
 * Notes:       We do this for one of two cases.  Either the host is busy
 *              and it cannot accept any more commands for the time being,
 *              or the device returned QUEUE_FULL and can accept no more
 *              commands.
 * Notes:       This could be called either from an interrupt context or a
 *              normal process context.
 */
int scsi_queue_insert(struct scsi_cmnd *cmd, int reason)
{
        struct Scsi_Host *host = cmd->device->host;
        struct scsi_device *device = cmd->device;

        SCSI_LOG_MLQUEUE(1,
                 printk("Inserting command %p into mlqueue\n", cmd));

        /*
         * We are inserting the command into the ml queue.  First, we
         * cancel the timer, so it doesn't time out.
         */
        scsi_delete_timer(cmd);

        /*
         * Next, set the appropriate busy bit for the device/host.
         *
         * If the host/device isn't busy, assume that something actually
         * completed, and that we should be able to queue a command now.
         *
         * Note that the prior mid-layer assumption that any host could
         * always queue at least one command is now broken.  The mid-layer
         * will implement a user specifiable stall (see
         * scsi_host.max_host_blocked and scsi_device.max_device_blocked)
         * if a command is requeued with no other commands outstanding
         * either for the device or for the host.
         */
        if (reason == SCSI_MLQUEUE_HOST_BUSY)
                host->host_blocked = host->max_host_blocked;
        else if (reason == SCSI_MLQUEUE_DEVICE_BUSY)
                device->device_blocked = device->max_device_blocked;

        /*
         * Register the fact that we own the thing for now.
         */
        cmd->state = SCSI_STATE_MLQUEUE;
        cmd->owner = SCSI_OWNER_MIDLEVEL;

        /*
         * Decrement the counters, since these commands are no longer
         * active on the host/device.
         */
        scsi_device_unbusy(device);

        /*
         * Insert this command at the head of the queue for it's device.
         * It will go before all other commands that are already in the queue.
         *
         * NOTE: there is magic here about the way the queue is plugged if
         * we have no outstanding commands.
         * 
         * Although this *doesn't* plug the queue, it does call the request
         * function.  The SCSI request function detects the blocked condition
         * and plugs the queue appropriately.
         */
        blk_insert_request(device->request_queue, cmd->request, 1, cmd, 1);
        return 0;
}

/*
 * Function:    scsi_do_req
 *
 * Purpose:     Queue a SCSI request
 *
 * Arguments:   sreq      - command descriptor.
 *              cmnd      - actual SCSI command to be performed.
 *              buffer    - data buffer.
 *              bufflen   - size of data buffer.
 *              done      - completion function to be run.
 *              timeout   - how long to let it run before timeout.
 *              retries   - number of retries we allow.
 *
 * Lock status: No locks held upon entry.
 *
 * Returns:     Nothing.
 *
 * Notes:       This function is only used for queueing requests for things
 *              like ioctls and character device requests - this is because
 *              we essentially just inject a request into the queue for the
 *              device.
 *
 *              In order to support the scsi_device_quiesce function, we
 *              now inject requests on the *head* of the device queue
 *              rather than the tail.
 */
void scsi_do_req(struct scsi_request *sreq, const void *cmnd,
                 void *buffer, unsigned bufflen,
                 void (*done)(struct scsi_cmnd *),
                 int timeout, int retries)
{
        /*
         * If the upper level driver is reusing these things, then
         * we should release the low-level block now.  Another one will
         * be allocated later when this request is getting queued.
         */
        __scsi_release_request(sreq);

        /*
         * Our own function scsi_done (which marks the host as not busy,
         * disables the timeout counter, etc) will be called by us or by the
         * scsi_hosts[host].queuecommand() function needs to also call
         * the completion function for the high level driver.
         */
        memcpy(sreq->sr_cmnd, cmnd, sizeof(sreq->sr_cmnd));
        sreq->sr_bufflen = bufflen;
        sreq->sr_buffer = buffer;
        sreq->sr_allowed = retries;
        sreq->sr_done = done;
        sreq->sr_timeout_per_command = timeout;

        if (sreq->sr_cmd_len == 0)
                sreq->sr_cmd_len = COMMAND_SIZE(sreq->sr_cmnd[0]);

        /*
         * head injection *required* here otherwise quiesce won't work
         */
        scsi_insert_special_req(sreq, 1);
}
 
static void scsi_wait_done(struct scsi_cmnd *cmd)
{
        struct request *req = cmd->request;
        struct request_queue *q = cmd->device->request_queue;
        unsigned long flags;

        req->rq_status = RQ_SCSI_DONE;  /* Busy, but indicate request done */

        spin_lock_irqsave(q->queue_lock, flags);
        if (blk_rq_tagged(req))
                blk_queue_end_tag(q, req);
        spin_unlock_irqrestore(q->queue_lock, flags);

        if (req->waiting)
                complete(req->waiting);
}

void scsi_wait_req(struct scsi_request *sreq, const void *cmnd, void *buffer,
                   unsigned bufflen, int timeout, int retries)
{
        DECLARE_COMPLETION(wait);
        
        sreq->sr_request->waiting = &wait;
        sreq->sr_request->rq_status = RQ_SCSI_BUSY;
        scsi_do_req(sreq, cmnd, buffer, bufflen, scsi_wait_done,
                        timeout, retries);
        generic_unplug_device(sreq->sr_device->request_queue);
        wait_for_completion(&wait);
        sreq->sr_request->waiting = NULL;
        if (sreq->sr_request->rq_status != RQ_SCSI_DONE)
                sreq->sr_result |= (DRIVER_ERROR << 24);

        __scsi_release_request(sreq);
}

/*
 * Function:    scsi_init_cmd_errh()
 *
 * Purpose:     Initialize cmd fields related to error handling.
 *
 * Arguments:   cmd     - command that is ready to be queued.
 *
 * Returns:     Nothing
 *
 * Notes:       This function has the job of initializing a number of
 *              fields related to error handling.   Typically this will
 *              be called once for each command, as required.
 */
static int scsi_init_cmd_errh(struct scsi_cmnd *cmd)
{
        cmd->owner = SCSI_OWNER_MIDLEVEL;
        cmd->serial_number = 0;
        cmd->serial_number_at_timeout = 0;
        cmd->abort_reason = 0;

        memset(cmd->sense_buffer, 0, sizeof cmd->sense_buffer);

        if (cmd->cmd_len == 0)
                cmd->cmd_len = COMMAND_SIZE(cmd->cmnd[0]);

        /*
         * We need saved copies of a number of fields - this is because
         * error handling may need to overwrite these with different values
         * to run different commands, and once error handling is complete,
         * we will need to restore these values prior to running the actual
         * command.
         */
        cmd->old_use_sg = cmd->use_sg;
        cmd->old_cmd_len = cmd->cmd_len;
        cmd->sc_old_data_direction = cmd->sc_data_direction;
        cmd->old_underflow = cmd->underflow;
        memcpy(cmd->data_cmnd, cmd->cmnd, sizeof(cmd->cmnd));
        cmd->buffer = cmd->request_buffer;
        cmd->bufflen = cmd->request_bufflen;
        cmd->internal_timeout = NORMAL_TIMEOUT;
        cmd->abort_reason = 0;

        return 1;
}

/*
 * Function:   scsi_setup_cmd_retry()
 *
 * Purpose:    Restore the command state for a retry
 *
 * Arguments:  cmd      - command to be restored
 *
 * Returns:    Nothing
 *
 * Notes:      Immediately prior to retrying a command, we need
 *             to restore certain fields that we saved above.
 */
void scsi_setup_cmd_retry(struct scsi_cmnd *cmd)
{
        memcpy(cmd->cmnd, cmd->data_cmnd, sizeof(cmd->data_cmnd));
        cmd->request_buffer = cmd->buffer;
        cmd->request_bufflen = cmd->bufflen;
        cmd->use_sg = cmd->old_use_sg;
        cmd->cmd_len = cmd->old_cmd_len;
        cmd->sc_data_direction = cmd->sc_old_data_direction;
        cmd->underflow = cmd->old_underflow;
}

void scsi_device_unbusy(struct scsi_device *sdev)
{
        struct Scsi_Host *shost = sdev->host;
        unsigned long flags;

        spin_lock_irqsave(shost->host_lock, flags);
        shost->host_busy--;
        if (unlikely(test_bit(SHOST_RECOVERY, &shost->shost_state) &&
                     shost->host_failed))
                scsi_eh_wakeup(shost);
        spin_unlock(shost->host_lock);
        spin_lock(&sdev->sdev_lock);
        sdev->device_busy--;
        spin_unlock_irqrestore(&sdev->sdev_lock, flags);
}

/*
 * Called for single_lun devices on IO completion. Clear starget_sdev_user,
 * and call blk_run_queue for all the scsi_devices on the target -
 * including current_sdev first.
 *
 * Called with *no* scsi locks held.
 */
static void scsi_single_lun_run(struct scsi_device *current_sdev)
{
        struct Scsi_Host *shost = current_sdev->host;
        struct scsi_device *sdev, *tmp;
        unsigned long flags;

        spin_lock_irqsave(shost->host_lock, flags);
        current_sdev->sdev_target->starget_sdev_user = NULL;
        spin_unlock_irqrestore(shost->host_lock, flags);

        /*
         * Call blk_run_queue for all LUNs on the target, starting with
         * current_sdev. We race with others (to set starget_sdev_user),
         * but in most cases, we will be first. Ideally, each LU on the
         * target would get some limited time or requests on the target.
         */
        blk_run_queue(current_sdev->request_queue);

        spin_lock_irqsave(shost->host_lock, flags);
        if (current_sdev->sdev_target->starget_sdev_user)
                goto out;
        list_for_each_entry_safe(sdev, tmp, &current_sdev->same_target_siblings,
                        same_target_siblings) {
                if (scsi_device_get(sdev))
                        continue;

                spin_unlock_irqrestore(shost->host_lock, flags);
                blk_run_queue(sdev->request_queue);
                spin_lock_irqsave(shost->host_lock, flags);
        
                scsi_device_put(sdev);
        }
 out:
        spin_unlock_irqrestore(shost->host_lock, flags);
}

/*
 * Function:    scsi_run_queue()
 *
 * Purpose:     Select a proper request queue to serve next
 *
 * Arguments:   q       - last request's queue
 *
 * Returns:     Nothing
 *
 * Notes:       The previous command was completely finished, start
 *              a new one if possible.
 */
static void scsi_run_queue(struct request_queue *q)
{
        struct scsi_device *sdev = q->queuedata;
        struct Scsi_Host *shost = sdev->host;
        unsigned long flags;

        if (sdev->single_lun)
                scsi_single_lun_run(sdev);

        spin_lock_irqsave(shost->host_lock, flags);
        while (!list_empty(&shost->starved_list) &&
               !shost->host_blocked && !shost->host_self_blocked &&
                !((shost->can_queue > 0) &&
                  (shost->host_busy >= shost->can_queue))) {
                /*
                 * As long as shost is accepting commands and we have
                 * starved queues, call blk_run_queue. scsi_request_fn
                 * drops the queue_lock and can add us back to the
                 * starved_list.
                 *
                 * host_lock protects the starved_list and starved_entry.
                 * scsi_request_fn must get the host_lock before checking
                 * or modifying starved_list or starved_entry.
                 */
                sdev = list_entry(shost->starved_list.next,
                                          struct scsi_device, starved_entry);
                list_del_init(&sdev->starved_entry);
                spin_unlock_irqrestore(shost->host_lock, flags);

                blk_run_queue(sdev->request_queue);

                spin_lock_irqsave(shost->host_lock, flags);
                if (unlikely(!list_empty(&sdev->starved_entry)))
                        /*
                         * sdev lost a race, and was put back on the
                         * starved list. This is unlikely but without this
                         * in theory we could loop forever.
                         */
                        break;
        }
        spin_unlock_irqrestore(shost->host_lock, flags);

        blk_run_queue(q);
}

/*
 * Function:    scsi_requeue_command()
 *
 * Purpose:     Handle post-processing of completed commands.
 *
 * Arguments:   q       - queue to operate on
 *              cmd     - command that may need to be requeued.
 *
 * Returns:     Nothing
 *
 * Notes:       After command completion, there may be blocks left
 *              over which weren't finished by the previous command
 *              this can be for a number of reasons - the main one is
 *              I/O errors in the middle of the request, in which case
 *              we need to request the blocks that come after the bad
 *              sector.
 */
static void scsi_requeue_command(struct request_queue *q, struct scsi_cmnd *cmd)
{
        cmd->request->flags &= ~REQ_DONTPREP;
        blk_insert_request(q, cmd->request, 1, cmd, 1);

        scsi_run_queue(q);
}

void scsi_next_command(struct scsi_cmnd *cmd)
{
        struct request_queue *q = cmd->device->request_queue;

        scsi_put_command(cmd);
        scsi_run_queue(q);
}

void scsi_run_host_queues(struct Scsi_Host *shost)
{
        struct scsi_device *sdev;

        shost_for_each_device(sdev, shost)
                scsi_run_queue(sdev->request_queue);
}

/*
 * Function:    scsi_end_request()
 *
 * Purpose:     Post-processing of completed commands called from interrupt
 *              handler or a bottom-half handler.
 *
 * Arguments:   cmd      - command that is complete.
 *              uptodate - 1 if I/O indicates success, 0 for I/O error.
 *              sectors  - number of sectors we want to mark.
 *              requeue  - indicates whether we should requeue leftovers.
 *              frequeue - indicates that if we release the command block
 *                         that the queue request function should be called.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       This is called for block device requests in order to
 *              mark some number of sectors as complete.
 * 
 *              We are guaranteeing that the request queue will be goosed
 *              at some point during this call.
 */
static struct scsi_cmnd *scsi_end_request(struct scsi_cmnd *cmd, int uptodate,
                                          int bytes, int requeue)
{
        request_queue_t *q = cmd->device->request_queue;
        struct request *req = cmd->request;
        unsigned long flags;

        /*
         * If there are blocks left over at the end, set up the command
         * to queue the remainder of them.
         */
        if (end_that_request_chunk(req, uptodate, bytes)) {
                int leftover = (req->hard_nr_sectors << 9);

                if (blk_pc_request(req))
                        leftover = req->data_len;

                /* kill remainder if no retrys */
                if (!uptodate && blk_noretry_request(req))
                        end_that_request_chunk(req, 0, leftover);
                else {
                        if (requeue)
                                /*
                                 * Bleah.  Leftovers again.  Stick the
                                 * leftovers in the front of the
                                 * queue, and goose the queue again.
                                 */
                                scsi_requeue_command(q, cmd);

                        return cmd;
                }
        }

        add_disk_randomness(req->rq_disk);

        spin_lock_irqsave(q->queue_lock, flags);
        if (blk_rq_tagged(req))
                blk_queue_end_tag(q, req);
        end_that_request_last(req);
        spin_unlock_irqrestore(q->queue_lock, flags);

        /*
         * This will goose the queue request function at the end, so we don't
         * need to worry about launching another command.
         */
        scsi_next_command(cmd);
        return NULL;
}

static struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, int gfp_mask)
{
        struct scsi_host_sg_pool *sgp;
        struct scatterlist *sgl;

        BUG_ON(!cmd->use_sg);

        switch (cmd->use_sg) {
        case 1 ... 8:
                cmd->sglist_len = 0;
                break;
        case 9 ... 16:
                cmd->sglist_len = 1;
                break;
        case 17 ... 32:
                cmd->sglist_len = 2;
                break;
#if (SCSI_MAX_PHYS_SEGMENTS > 32)
        case 33 ... 64:
                cmd->sglist_len = 3;
                break;
#if (SCSI_MAX_PHYS_SEGMENTS > 64)
        case 65 ... 128:
                cmd->sglist_len = 4;
                break;
#if (SCSI_MAX_PHYS_SEGMENTS  > 128)
        case 129 ... 256:
                cmd->sglist_len = 5;
                break;
#endif
#endif
#endif
        default:
                return NULL;
        }

        sgp = scsi_sg_pools + cmd->sglist_len;
        sgl = mempool_alloc(sgp->pool, gfp_mask);
        if (sgl)
                memset(sgl, 0, sgp->size);
        return sgl;
}

static void scsi_free_sgtable(struct scatterlist *sgl, int index)
{
        struct scsi_host_sg_pool *sgp;

        BUG_ON(index > SG_MEMPOOL_NR);

        sgp = scsi_sg_pools + index;
        mempool_free(sgl, sgp->pool);
}

/*
 * Function:    scsi_release_buffers()
 *
 * Purpose:     Completion processing for block device I/O requests.
 *
 * Arguments:   cmd     - command that we are bailing.
 *
 * Lock status: Assumed that no lock is held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       In the event that an upper level driver rejects a
 *              command, we must release resources allocated during
 *              the __init_io() function.  Primarily this would involve
 *              the scatter-gather table, and potentially any bounce
 *              buffers.
 */
static void scsi_release_buffers(struct scsi_cmnd *cmd)
{
        struct request *req = cmd->request;

        /*
         * Free up any indirection buffers we allocated for DMA purposes. 
         */
        if (cmd->use_sg)
                scsi_free_sgtable(cmd->request_buffer, cmd->sglist_len);
        else if (cmd->request_buffer != req->buffer)
                kfree(cmd->request_buffer);

        /*
         * Zero these out.  They now point to freed memory, and it is
         * dangerous to hang onto the pointers.
         */
        cmd->buffer  = NULL;
        cmd->bufflen = 0;
        cmd->request_buffer = NULL;
        cmd->request_bufflen = 0;
}

/*
 * Function:    scsi_io_completion()
 *
 * Purpose:     Completion processing for block device I/O requests.
 *
 * Arguments:   cmd   - command that is finished.
 *
 * Lock status: Assumed that no lock is held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       This function is matched in terms of capabilities to
 *              the function that created the scatter-gather list.
 *              In other words, if there are no bounce buffers
 *              (the normal case for most drivers), we don't need
 *              the logic to deal with cleaning up afterwards.
 *
 *              We must do one of several things here:
 *
 *              a) Call scsi_end_request.  This will finish off the
 *                 specified number of sectors.  If we are done, the
 *                 command block will be released, and the queue
 *                 function will be goosed.  If we are not done, then
 *                 scsi_end_request will directly goose the queue.
 *
 *              b) We can just use scsi_requeue_command() here.  This would
 *                 be used if we just wanted to retry, for example.
 */
void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes,
                        unsigned int block_bytes)
{
        int result = cmd->result;
        int this_count = cmd->bufflen;
        request_queue_t *q = cmd->device->request_queue;
        struct request *req = cmd->request;
        int clear_errors = 1;

        /*
         * Free up any indirection buffers we allocated for DMA purposes. 
         * For the case of a READ, we need to copy the data out of the
         * bounce buffer and into the real buffer.
         */
        if (cmd->use_sg)
                scsi_free_sgtable(cmd->buffer, cmd->sglist_len);
        else if (cmd->buffer != req->buffer) {
                if (rq_data_dir(req) == READ) {
                        unsigned long flags;
                        char *to = bio_kmap_irq(req->bio, &flags);
                        memcpy(to, cmd->buffer, cmd->bufflen);
                        bio_kunmap_irq(to, &flags);
                }
                kfree(cmd->buffer);
        }

        if (blk_pc_request(req)) { /* SG_IO ioctl from block level */
                req->errors = (driver_byte(result) & DRIVER_SENSE) ?
                              (CHECK_CONDITION << 1) : (result & 0xff);
                if (result) {
                        clear_errors = 0;
                        if (cmd->sense_buffer[0] & 0x70) {
                                int len = 8 + cmd->sense_buffer[7];

                                if (len > SCSI_SENSE_BUFFERSIZE)
                                        len = SCSI_SENSE_BUFFERSIZE;
                                memcpy(req->sense, cmd->sense_buffer,  len);
                                req->sense_len = len;
                        }
                } else
                        req->data_len -= cmd->bufflen;
        }

        /*
         * Zero these out.  They now point to freed memory, and it is
         * dangerous to hang onto the pointers.
         */
        cmd->buffer  = NULL;
        cmd->bufflen = 0;
        cmd->request_buffer = NULL;
        cmd->request_bufflen = 0;

        /*
         * Next deal with any sectors which we were able to correctly
         * handle.
         */
        if (good_bytes >= 0) {
                SCSI_LOG_HLCOMPLETE(1, printk("%ld sectors total, %d bytes done.\n",
                                              req->nr_sectors, good_bytes));
                SCSI_LOG_HLCOMPLETE(1, printk("use_sg is %d\n", cmd->use_sg));

                if (clear_errors)
                        req->errors = 0;
                /*
                 * If multiple sectors are requested in one buffer, then
                 * they will have been finished off by the first command.
                 * If not, then we have a multi-buffer command.
                 *
                 * If block_bytes != 0, it means we had a medium error
                 * of some sort, and that we want to mark some number of
                 * sectors as not uptodate.  Thus we want to inhibit
                 * requeueing right here - we will requeue down below
                 * when we handle the bad sectors.
                 */
                cmd = scsi_end_request(cmd, 1, good_bytes, result == 0);

                /*
                 * If the command completed without error, then either finish off the
                 * rest of the command, or start a new one.
                 */
                if (result == 0 || cmd == NULL ) {
                        return;
                }
        }
        /*
         * Now, if we were good little boys and girls, Santa left us a request
         * sense buffer.  We can extract information from this, so we
         * can choose a block to remap, etc.
         */
        if (driver_byte(result) != 0) {
                if ((cmd->sense_buffer[0] & 0x7f) == 0x70) {
                        /*
                         * If the device is in the process of becoming ready,
                         * retry.
                         */
                        if (cmd->sense_buffer[12] == 0x04 &&
                            cmd->sense_buffer[13] == 0x01) {
                                scsi_requeue_command(q, cmd);
                                return;
                        }
                        if ((cmd->sense_buffer[2] & 0xf) == UNIT_ATTENTION) {
                                if (cmd->device->removable) {
                                        /* detected disc change.  set a bit 
                                         * and quietly refuse further access.
                                         */
                                        cmd->device->changed = 1;
                                        cmd = scsi_end_request(cmd, 0,
                                                        this_count, 1);
                                        return;
                                } else {
                                        /*
                                        * Must have been a power glitch, or a
                                        * bus reset.  Could not have been a
                                        * media change, so we just retry the
                                        * request and see what happens.  
                                        */
                                        scsi_requeue_command(q, cmd);
                                        return;
                                }
                        }
                }
                /*
                 * If we had an ILLEGAL REQUEST returned, then we may have
                 * performed an unsupported command.  The only thing this
                 * should be would be a ten byte read where only a six byte
                 * read was supported.  Also, on a system where READ CAPACITY
                 * failed, we may have read past the end of the disk.
                 */

                switch (cmd->sense_buffer[2]) {
                case ILLEGAL_REQUEST:
                        if (cmd->device->use_10_for_rw &&
                            (cmd->cmnd[0] == READ_10 ||
                             cmd->cmnd[0] == WRITE_10)) {
                                cmd->device->use_10_for_rw = 0;
                                /*
                                 * This will cause a retry with a 6-byte
                                 * command.
                                 */
                                scsi_requeue_command(q, cmd);
                                result = 0;
                        } else {
                                cmd = scsi_end_request(cmd, 0, this_count, 1);
                                return;
                        }
                        break;
                case NOT_READY:
                        printk(KERN_INFO "Device %s not ready.\n",
                               req->rq_disk ? req->rq_disk->disk_name : "");
                        cmd = scsi_end_request(cmd, 0, this_count, 1);
                        return;
                        break;
                case MEDIUM_ERROR:
                case VOLUME_OVERFLOW:
                        printk("scsi%d: ERROR on channel %d, id %d, lun %d, CDB: ",
                               cmd->device->host->host_no, (int) cmd->device->channel,
                               (int) cmd->device->id, (int) cmd->device->lun);
                        print_command(cmd->data_cmnd);
                        print_sense("", cmd);
                        cmd = scsi_end_request(cmd, 0, block_bytes, 1);
                        return;
                default:
                        break;
                }
        }                       /* driver byte != 0 */
        if (host_byte(result) == DID_RESET) {
                /*
                 * Third party bus reset or reset for error
                 * recovery reasons.  Just retry the request
                 * and see what happens.  
                 */
                scsi_requeue_command(q, cmd);
                return;
        }
        if (result) {
                printk("SCSI error : <%d %d %d %d> return code = 0x%x\n",
                       cmd->device->host->host_no,
                       cmd->device->channel,
                       cmd->device->id,
                       cmd->device->lun, result);

                if (driver_byte(result) & DRIVER_SENSE)
                        print_sense("", cmd);
                /*
                 * Mark a single buffer as not uptodate.  Queue the remainder.
                 * We sometimes get this cruft in the event that a medium error
                 * isn't properly reported.
                 */
                block_bytes = req->hard_cur_sectors << 9;
                if (!block_bytes)
                        block_bytes = req->data_len;
                cmd = scsi_end_request(cmd, 0, block_bytes, 1);
        }
}

/*
 * Function:    scsi_init_io()
 *
 * Purpose:     SCSI I/O initialize function.
 *
 * Arguments:   cmd   - Command descriptor we wish to initialize
 *
 * Returns:     0 on success
 *              BLKPREP_DEFER if the failure is retryable
 *              BLKPREP_KILL if the failure is fatal
 */
static int scsi_init_io(struct scsi_cmnd *cmd)
{
        struct request     *req = cmd->request;
        struct scatterlist *sgpnt;
        int                count;

        /*
         * if this is a rq->data based REQ_BLOCK_PC, setup for a non-sg xfer
         */
        if ((req->flags & REQ_BLOCK_PC) && !req->bio) {
                cmd->request_bufflen = req->data_len;
                cmd->request_buffer = req->data;
                req->buffer = req->data;
                cmd->use_sg = 0;
                return 0;
        }

        /*
         * we used to not use scatter-gather for single segment request,
         * but now we do (it makes highmem I/O easier to support without
         * kmapping pages)
         */
        cmd->use_sg = req->nr_phys_segments;

        /*
         * if sg table allocation fails, requeue request later.
         */
        sgpnt = scsi_alloc_sgtable(cmd, GFP_ATOMIC);
        if (unlikely(!sgpnt)) {
                req->flags |= REQ_SPECIAL;
                return BLKPREP_DEFER;
        }

        cmd->request_buffer = (char *) sgpnt;
        cmd->request_bufflen = req->nr_sectors << 9;
        if (blk_pc_request(req))
                cmd->request_bufflen = req->data_len;
        req->buffer = NULL;

        /* 
         * Next, walk the list, and fill in the addresses and sizes of
         * each segment.
         */
        count = blk_rq_map_sg(req->q, req, cmd->request_buffer);

        /*
         * mapped well, send it off
         */
        if (likely(count <= cmd->use_sg)) {
                cmd->use_sg = count;
                return 0;
        }

        printk(KERN_ERR "Incorrect number of segments after building list\n");
        printk(KERN_ERR "counted %d, received %d\n", count, cmd->use_sg);
        printk(KERN_ERR "req nr_sec %lu, cur_nr_sec %u\n", req->nr_sectors,
                        req->current_nr_sectors);

        /* release the command and kill it */
        scsi_release_buffers(cmd);
        scsi_put_command(cmd);
        return BLKPREP_KILL;
}

static int scsi_prep_fn(struct request_queue *q, struct request *req)
{
        struct scsi_device *sdev = q->queuedata;
        struct scsi_cmnd *cmd;
        int specials_only = 0;

        /*
         * Just check to see if the device is online.  If it isn't, we
         * refuse to process any commands.  The device must be brought
         * online before trying any recovery commands
         */
        if (unlikely(!scsi_device_online(sdev))) {
                printk(KERN_ERR "scsi%d (%d:%d): rejecting I/O to offline device\n",
                       sdev->host->host_no, sdev->id, sdev->lun);
                return BLKPREP_KILL;
        }
        if (unlikely(sdev->sdev_state != SDEV_RUNNING)) {
                /* OK, we're not in a running state don't prep
                 * user commands */
                if (sdev->sdev_state == SDEV_DEL) {
                        /* Device is fully deleted, no commands
                         * at all allowed down */
                        printk(KERN_ERR "scsi%d (%d:%d): rejecting I/O to dead device\n",
                               sdev->host->host_no, sdev->id, sdev->lun);
                        return BLKPREP_KILL;
                }
                /* OK, we only allow special commands (i.e. not
                 * user initiated ones */
                specials_only = sdev->sdev_state;
        }

        /*
         * Find the actual device driver associated with this command.
         * The SPECIAL requests are things like character device or
         * ioctls, which did not originate from ll_rw_blk.  Note that
         * the special field is also used to indicate the cmd for
         * the remainder of a partially fulfilled request that can 
         * come up when there is a medium error.  We have to treat
         * these two cases differently.  We differentiate by looking
         * at request->cmd, as this tells us the real story.
         */
        if (req->flags & REQ_SPECIAL) {
                struct scsi_request *sreq = req->special;

                if (sreq->sr_magic == SCSI_REQ_MAGIC) {
                        cmd = scsi_get_command(sreq->sr_device, GFP_ATOMIC);
                        if (unlikely(!cmd))
                                goto defer;
                        scsi_init_cmd_from_req(cmd, sreq);
                } else
                        cmd = req->special;
        } else if (req->flags & (REQ_CMD | REQ_BLOCK_PC)) {

                if(unlikely(specials_only)) {
                        if(specials_only == SDEV_QUIESCE)
                                return BLKPREP_DEFER;
                        
                        printk(KERN_ERR "scsi%d (%d:%d): rejecting I/O to device being removed\n",
                               sdev->host->host_no, sdev->id, sdev->lun);
                        return BLKPREP_KILL;
                }
                        
                        
                /*
                 * Now try and find a command block that we can use.
                 */
                if (!req->special) {
                        cmd = scsi_get_command(sdev, GFP_ATOMIC);
                        if (unlikely(!cmd))
                                goto defer;
                } else
                        cmd = req->special;
                
                /* pull a tag out of the request if we have one */
                cmd->tag = req->tag;
        } else {
                blk_dump_rq_flags(req, "SCSI bad req");
                return BLKPREP_KILL;
        }
        
        /* note the overloading of req->special.  When the tag
         * is active it always means cmd.  If the tag goes
         * back for re-queueing, it may be reset */
        req->special = cmd;
        cmd->request = req;
        
        /*
         * FIXME: drop the lock here because the functions below
         * expect to be called without the queue lock held.  Also,
         * previously, we dequeued the request before dropping the
         * lock.  We hope REQ_STARTED prevents anything untoward from
         * happening now.
         */
        if (req->flags & (REQ_CMD | REQ_BLOCK_PC)) {
                struct scsi_driver *drv;
                int ret;

                /*
                 * This will do a couple of things:
                 *  1) Fill in the actual SCSI command.
                 *  2) Fill in any other upper-level specific fields
                 * (timeout).
                 *
                 * If this returns 0, it means that the request failed
                 * (reading past end of disk, reading offline device,
                 * etc).   This won't actually talk to the device, but
                 * some kinds of consistency checking may cause the     
                 * request to be rejected immediately.
                 */

                /* 
                 * This sets up the scatter-gather table (allocating if
                 * required).
                 */
                ret = scsi_init_io(cmd);
                if (ret)        /* BLKPREP_KILL return also releases the command */
                        return ret;
                
                /*
                 * Initialize the actual SCSI command for this request.
                 */
                drv = *(struct scsi_driver **)req->rq_disk->private_data;
                if (unlikely(!drv->init_command(cmd))) {
                        scsi_release_buffers(cmd);
                        scsi_put_command(cmd);
                        return BLKPREP_KILL;
                }
        }

        /*
         * The request is now prepped, no need to come back here
         */
        req->flags |= REQ_DONTPREP;
        return BLKPREP_OK;

 defer:
        /* If we defer, the elv_next_request() returns NULL, but the
         * queue must be restarted, so we plug here if no returning
         * command will automatically do that. */
        if (sdev->device_busy == 0)
                blk_plug_device(q);
        return BLKPREP_DEFER;
}

/*
 * scsi_dev_queue_ready: if we can send requests to sdev, return 1 else
 * return 0.
 *
 * Called with the queue_lock held.
 */
static inline int scsi_dev_queue_ready(struct request_queue *q,
                                  struct scsi_device *sdev)
{
        if (sdev->device_busy >= sdev->queue_depth)
                return 0;
        if (sdev->device_busy == 0 && sdev->device_blocked) {
                /*
                 * unblock after device_blocked iterates to zero
                 */
                if (--sdev->device_blocked == 0) {
                        SCSI_LOG_MLQUEUE(3,
                                printk("scsi%d (%d:%d) unblocking device at"
                                       " zero depth\n", sdev->host->host_no,
                                       sdev->id, sdev->lun));
                } else {
                        blk_plug_device(q);
                        return 0;
                }
        }
        if (sdev->device_blocked)
                return 0;

        return 1;
}

/*
 * scsi_host_queue_ready: if we can send requests to shost, return 1 else
 * return 0. We must end up running the queue again whenever 0 is
 * returned, else IO can hang.
 *
 * Called with host_lock held.
 */
static inline int scsi_host_queue_ready(struct request_queue *q,
                                   struct Scsi_Host *shost,
                                   struct scsi_device *sdev)
{
        if (test_bit(SHOST_RECOVERY, &shost->shost_state))
                return 0;
        if (shost->host_busy == 0 && shost->host_blocked) {
                /*
                 * unblock after host_blocked iterates to zero
                 */
                if (--shost->host_blocked == 0) {
                        SCSI_LOG_MLQUEUE(3,
                                printk("scsi%d unblocking host at zero depth\n",
                                        shost->host_no));
                } else {
                        blk_plug_device(q);
                        return 0;
                }
        }
        if ((shost->can_queue > 0 && shost->host_busy >= shost->can_queue) ||
            shost->host_blocked || shost->host_self_blocked) {
                if (list_empty(&sdev->starved_entry))
                        list_add_tail(&sdev->starved_entry, &shost->starved_list);
                return 0;
        }

        /* We're OK to process the command, so we can't be starved */
        if (!list_empty(&sdev->starved_entry))
                list_del_init(&sdev->starved_entry);

        return 1;
}

/*
 * Function:    scsi_request_fn()
 *
 * Purpose:     Main strategy routine for SCSI.
 *
 * Arguments:   q       - Pointer to actual queue.
 *
 * Returns:     Nothing
 *
 * Lock status: IO request lock assumed to be held when called.
 */
static void scsi_request_fn(struct request_queue *q)
{
        struct scsi_device *sdev = q->queuedata;
        struct Scsi_Host *shost = sdev->host;
        struct scsi_cmnd *cmd;
        struct request *req;

        if(!get_device(&sdev->sdev_gendev))
                /* We must be tearing the block queue down already */
                return;

        /*
         * To start with, we keep looping until the queue is empty, or until
         * the host is no longer able to accept any more requests.
         */
        while (!blk_queue_plugged(q)) {
                int rtn;
                /*
                 * get next queueable request.  We do this early to make sure
                 * that the request is fully prepared even if we cannot 
                 * accept it.
                 */
                req = elv_next_request(q);
                if (!req || !scsi_dev_queue_ready(q, sdev))
                        break;

                if (unlikely(!scsi_device_online(sdev))) {
                        printk(KERN_ERR "scsi%d (%d:%d): rejecting I/O to offline device\n",
                               sdev->host->host_no, sdev->id, sdev->lun);
                        blkdev_dequeue_request(req);
                        req->flags |= REQ_QUIET;
                        while (end_that_request_first(req, 0, req->nr_sectors))
                                ;
                        end_that_request_last(req);
                        continue;
                }


                /*
                 * Remove the request from the request list.
                 */
                if (!(blk_queue_tagged(q) && !blk_queue_start_tag(q, req)))
                        blkdev_dequeue_request(req);
                sdev->device_busy++;

                spin_unlock(q->queue_lock);
                spin_lock(shost->host_lock);

                if (!scsi_host_queue_ready(q, shost, sdev))
                        goto not_ready;
                if (sdev->single_lun) {
                        if (sdev->sdev_target->starget_sdev_user &&
                            sdev->sdev_target->starget_sdev_user != sdev)
                                goto not_ready;
                        sdev->sdev_target->starget_sdev_user = sdev;
                }
                shost->host_busy++;

                /*
                 * XXX(hch): This is rather suboptimal, scsi_dispatch_cmd will
                 *              take the lock again.
                 */
                spin_unlock_irq(shost->host_lock);

                cmd = req->special;
                if (unlikely(cmd == NULL)) {
                        printk(KERN_CRIT "impossible request in %s.\n"
                                         "please mail a stack trace to "
                                         "linux-scsi@vger.kernel.org",
                                         __FUNCTION__);
                        BUG();
                }

                /*
                 * Finally, initialize any error handling parameters, and set up
                 * the timers for timeouts.
                 */
                scsi_init_cmd_errh(cmd);

                /*
                 * Dispatch the command to the low-level driver.
                 */
                rtn = scsi_dispatch_cmd(cmd);
                spin_lock_irq(q->queue_lock);
                if(rtn) {
                        /* we're refusing the command; because of
                         * the way locks get dropped, we need to 
                         * check here if plugging is required */
                        if(sdev->device_busy == 0)
                                blk_plug_device(q);

                        break;
                }
        }

        goto out;

 not_ready:
        spin_unlock_irq(shost->host_lock);

        /*
         * lock q, handle tag, requeue req, and decrement device_busy. We
         * must return with queue_lock held.
         *
         * Decrementing device_busy without checking it is OK, as all such
         * cases (host limits or settings) should run the queue at some
         * later time.
         */
        spin_lock_irq(q->queue_lock);
        blk_requeue_request(q, req);
        sdev->device_busy--;
        if(sdev->device_busy == 0)
                blk_plug_device(q);
 out:
        /* must be careful here...if we trigger the ->remove() function
         * we cannot be holding the q lock */
        spin_unlock_irq(q->queue_lock);
        put_device(&sdev->sdev_gendev);
        spin_lock_irq(q->queue_lock);
}

u64 scsi_calculate_bounce_limit(struct Scsi_Host *shost)
{
        struct device *host_dev;

        if (shost->unchecked_isa_dma)
                return BLK_BOUNCE_ISA;

        host_dev = scsi_get_device(shost);
        if (PCI_DMA_BUS_IS_PHYS && host_dev && host_dev->dma_mask)
                return *host_dev->dma_mask;

        /*
         * Platforms with virtual-DMA translation
         * hardware have no practical limit.
         */
        return BLK_BOUNCE_ANY;
}

struct request_queue *scsi_alloc_queue(struct scsi_device *sdev)
{
        struct Scsi_Host *shost = sdev->host;
        struct request_queue *q;

        q = blk_init_queue(scsi_request_fn, &sdev->sdev_lock);
        if (!q)
                return NULL;

        blk_queue_prep_rq(q, scsi_prep_fn);

        blk_queue_max_hw_segments(q, shost->sg_tablesize);
        blk_queue_max_phys_segments(q, SCSI_MAX_PHYS_SEGMENTS);
        blk_queue_max_sectors(q, shost->max_sectors);
        blk_queue_bounce_limit(q, scsi_calculate_bounce_limit(shost));
        blk_queue_segment_boundary(q, shost->dma_boundary);
 
        if (!shost->use_clustering)
                clear_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);
        return q;
}

void scsi_free_queue(struct request_queue *q)
{
        blk_cleanup_queue(q);
}

/*
 * Function:    scsi_block_requests()
 *
 * Purpose:     Utility function used by low-level drivers to prevent further
 *              commands from being queued to the device.
 *
 * Arguments:   shost       - Host in question
 *
 * Returns:     Nothing
 *
 * Lock status: No locks are assumed held.
 *
 * Notes:       There is no timer nor any other means by which the requests
 *              get unblocked other than the low-level driver calling
 *              scsi_unblock_requests().
 */
void scsi_block_requests(struct Scsi_Host *shost)
{
        shost->host_self_blocked = 1;
}

/*
 * Function:    scsi_unblock_requests()
 *
 * Purpose:     Utility function used by low-level drivers to allow further
 *              commands from being queued to the device.
 *
 * Arguments:   shost       - Host in question
 *
 * Returns:     Nothing
 *
 * Lock status: No locks are assumed held.
 *
 * Notes:       There is no timer nor any other means by which the requests
 *              get unblocked other than the low-level driver calling
 *              scsi_unblock_requests().
 *
 *              This is done as an API function so that changes to the
 *              internals of the scsi mid-layer won't require wholesale
 *              changes to drivers that use this feature.
 */
void scsi_unblock_requests(struct Scsi_Host *shost)
{
        shost->host_self_blocked = 0;
        scsi_run_host_queues(shost);
}

int __init scsi_init_queue(void)
{
        int i;

        for (i = 0; i < SG_MEMPOOL_NR; i++) {
                struct scsi_host_sg_pool *sgp = scsi_sg_pools + i;
                int size = sgp->size * sizeof(struct scatterlist);

                sgp->slab = kmem_cache_create(sgp->name, size, 0,
                                SLAB_HWCACHE_ALIGN, NULL, NULL);
                if (!sgp->slab) {
                        printk(KERN_ERR "SCSI: can't init sg slab %s\n",
                                        sgp->name);
                }

                sgp->pool = mempool_create(SG_MEMPOOL_SIZE,
                                mempool_alloc_slab, mempool_free_slab,
                                sgp->slab);
                if (!sgp->pool) {
                        printk(KERN_ERR "SCSI: can't init sg mempool %s\n",
                                        sgp->name);
                }
        }

        return 0;
}

void scsi_exit_queue(void)
{
        int i;

        for (i = 0; i < SG_MEMPOOL_NR; i++) {
                struct scsi_host_sg_pool *sgp = scsi_sg_pools + i;
                mempool_destroy(sgp->pool);
                kmem_cache_destroy(sgp->slab);
        }
}
/**
 *      __scsi_mode_sense - issue a mode sense, falling back from 10 to 
 *              six bytes if necessary.
 *      @sreq:  SCSI request to fill in with the MODE_SENSE
 *      @dbd:   set if mode sense will allow block descriptors to be returned
 *      @modepage: mode page being requested
 *      @buffer: request buffer (may not be smaller than eight bytes)
 *      @len:   length of request buffer.
 *      @timeout: command timeout
 *      @retries: number of retries before failing
 *      @data: returns a structure abstracting the mode header data
 *
 *      Returns zero if unsuccessful, or the header offset (either 4
 *      or 8 depending on whether a six or ten byte command was
 *      issued) if successful.
 **/
int
__scsi_mode_sense(struct scsi_request *sreq, int dbd, int modepage,
                  unsigned char *buffer, int len, int timeout, int retries,
                  struct scsi_mode_data *data) {
        unsigned char cmd[12];
        int use_10_for_ms;
        int header_length;

        memset(data, 0, sizeof(*data));
        memset(&cmd[0], 0, 12);
        cmd[1] = dbd & 0x18;    /* allows DBD and LLBA bits */
        cmd[2] = modepage;

 retry:
        use_10_for_ms = sreq->sr_device->use_10_for_ms;

        if (use_10_for_ms) {
                if (len < 8)
                        len = 8;

                cmd[0] = MODE_SENSE_10;
                cmd[8] = len;
                header_length = 8;
        } else {
                if (len < 4)
                        len = 4;

                cmd[0] = MODE_SENSE;
                cmd[4] = len;
                header_length = 4;
        }

        sreq->sr_cmd_len = 0;
        sreq->sr_sense_buffer[0] = 0;
        sreq->sr_sense_buffer[2] = 0;
        sreq->sr_data_direction = DMA_FROM_DEVICE;

        memset(buffer, 0, len);

        scsi_wait_req(sreq, cmd, buffer, len, timeout, retries);

        /* This code looks awful: what it's doing is making sure an
         * ILLEGAL REQUEST sense return identifies the actual command
         * byte as the problem.  MODE_SENSE commands can return
         * ILLEGAL REQUEST if the code page isn't supported */
        if (use_10_for_ms && ! scsi_status_is_good(sreq->sr_result) &&
            (driver_byte(sreq->sr_result) & DRIVER_SENSE) &&
            sreq->sr_sense_buffer[2] == ILLEGAL_REQUEST &&
            (sreq->sr_sense_buffer[4] & 0x40) == 0x40 &&
            sreq->sr_sense_buffer[5] == 0 &&
            sreq->sr_sense_buffer[6] == 0 ) {
                sreq->sr_device->use_10_for_ms = 0;
                goto retry;
        }

        if(scsi_status_is_good(sreq->sr_result)) {
                data->header_length = header_length;
                if(use_10_for_ms) {
                        data->length = buffer[0]*256 + buffer[1] + 2;
                        data->medium_type = buffer[2];
                        data->device_specific = buffer[3];
                        data->longlba = buffer[4] & 0x01;
                        data->block_descriptor_length = buffer[6]*256
                                + buffer[7];
                } else {
                        data->length = buffer[0] + 1;
                        data->medium_type = buffer[1];
                        data->device_specific = buffer[2];
                        data->block_descriptor_length = buffer[3];
                }
        }

        return sreq->sr_result;
}

/**
 *      scsi_mode_sense - issue a mode sense, falling back from 10 to 
 *              six bytes if necessary.
 *      @sdev:  scsi device to send command to.
 *      @dbd:   set if mode sense will disable block descriptors in the return
 *      @modepage: mode page being requested
 *      @buffer: request buffer (may not be smaller than eight bytes)
 *      @len:   length of request buffer.
 *      @timeout: command timeout
 *      @retries: number of retries before failing
 *
 *      Returns zero if unsuccessful, or the header offset (either 4
 *      or 8 depending on whether a six or ten byte command was
 *      issued) if successful.
 **/
int
scsi_mode_sense(struct scsi_device *sdev, int dbd, int modepage,
                unsigned char *buffer, int len, int timeout, int retries,
                struct scsi_mode_data *data)
{
        struct scsi_request *sreq = scsi_allocate_request(sdev, GFP_KERNEL);
        int ret;

        if (!sreq)
                return -1;

        ret = __scsi_mode_sense(sreq, dbd, modepage, buffer, len,
                                timeout, retries, data);

        scsi_release_request(sreq);

        return ret;
}

/**
 *      scsi_device_set_state - Take the given device through the device
 *              state model.
 *      @sdev:  scsi device to change the state of.
 *      @state: state to change to.
 *
 *      Returns zero if unsuccessful or an error if the requested 
 *      transition is illegal.
 **/
int
scsi_device_set_state(struct scsi_device *sdev, enum scsi_device_state state)
{
        enum scsi_device_state oldstate = sdev->sdev_state;

        if (state == oldstate)
                return 0;

        switch (state) {
        case SDEV_CREATED:
                /* There are no legal states that come back to
                 * created.  This is the manually initialised start
                 * state */
                goto illegal;
                        
        case SDEV_RUNNING:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_OFFLINE:
                case SDEV_QUIESCE:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_QUIESCE:
                switch (oldstate) {
                case SDEV_RUNNING:
                case SDEV_OFFLINE:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_OFFLINE:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_RUNNING:
                case SDEV_QUIESCE:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_CANCEL:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_RUNNING:
                case SDEV_OFFLINE:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_DEL:
                switch (oldstate) {
                case SDEV_CANCEL:
                        break;
                default:
                        goto illegal;
                }
                break;

        }
        sdev->sdev_state = state;
        return 0;

 illegal:
        dev_printk(KERN_ERR, &sdev->sdev_gendev,
                   "Illegal state transition %s->%s\n",
                   scsi_device_state_name(oldstate),
                   scsi_device_state_name(state));
        WARN_ON(1);
        return -EINVAL;
}
EXPORT_SYMBOL(scsi_device_set_state);

/**
 *      scsi_device_quiesce - Block user issued commands.
 *      @sdev:  scsi device to quiesce.
 *
 *      This works by trying to transition to the SDEV_QUIESCE state
 *      (which must be a legal transition).  When the device is in this
 *      state, only special requests will be accepted, all others will
 *      be deferred.  Since special requests may also be requeued requests,
 *      a successful return doesn't guarantee the device will be 
 *      totally quiescent.
 *
 *      Must be called with user context, may sleep.
 *
 *      Returns zero if unsuccessful or an error if not.
 **/
int
scsi_device_quiesce(struct scsi_device *sdev)
{
        int err = scsi_device_set_state(sdev, SDEV_QUIESCE);
        if (err)
                return err;

        scsi_run_queue(sdev->request_queue);
        while (sdev->device_busy) {
                schedule_timeout(HZ/5);
                scsi_run_queue(sdev->request_queue);
        }
        return 0;
}
EXPORT_SYMBOL(scsi_device_quiesce);

/**
 *      scsi_device_resume - Restart user issued commands to a quiesced device.
 *      @sdev:  scsi device to resume.
 *
 *      Moves the device from quiesced back to running and restarts the
 *      queues.
 *
 *      Must be called with user context, may sleep.
 **/
void
scsi_device_resume(struct scsi_device *sdev)
{
        if(scsi_device_set_state(sdev, SDEV_RUNNING))
                return;
        scsi_run_queue(sdev->request_queue);
}
EXPORT_SYMBOL(scsi_device_resume);